Such burners normally comprise a burner head supported on a skirt assembly. A combustible mixture of gas and air is supplied to the interior of the skirt, and emerges at combustion ports round the top of the burner head, where it burns.
In many burners, particularly burners using natural gas, the velocity of the combustible mixture as it emerges from the ports is such that the flames tend to lift off from the ports and may be extinguished. One method of preventing this, which is employed in the present invention, is to divert a proportion of the combustible gas/air mixture though large slots at the base of the burner, from which it emerges at a much lower velocity, forming a retention flame which prevents the flames at the burner ports above it from lifting off.
The volume of gas passed to the retention flame is normally between 7 and 23% of the total gas air mixture passing into the burner. Control of this proportion is important, since if the flow is too large the velocity may be such at the retention flame itself lifts off, while if it is too small the retention flame may be insufficient to prevent lifting off of the flames at the burner ports.
The gap between the burner base and the top of the skirt assembly forms a metering orifice which controls the flow of combustible gas to the retention flame. The need for accuracy in metering this flow requires that the top surface of the burner skirt should be very accurately formed, and consequently the skirt has hitherto been made by diecasting in aluminium or aluminium alloy. This is an expensive and inconvenient method, and moreover has the disadvantage that the skirt discolours in use and cannot be protected by coating processes, such as vitreous enamelling.